Broadcast radio frequency signals, such as television signals, travel through the air at great speed. The delay between the time a broadcast signal leaves a transmitter and arrives at a receiver is referred to as a propagation delay. Broadcast signals are also reflected by solid surfaces in much the same way that light is reflected. These reflections create multipath distortion or "ghosting". There is a direct or main path between a transmitter and a receiver which is characterized by a given propagation delay and one or more indirect paths, consisting of reflected signals, that have proportionately longer propagation delays. The receiver therefore receives two or more nearly identical signals (the ghost is weaker) that are displaced in time. Ghost signals generally detract significantly from the video display.
A solution to multipath television signals, by way of ghost cancellation or ghost minimization has long been a challenge to the television industry. The cancellation systems that have been proposed typically involve the use of adaptive equalizers that generate a signal from the main signal which can be used to cancel the ghost. Adaptive equalizers require a large number of real multipliers (between 250 and 500) to provide adequate ghost reduction performance in the receiver. At this level of complexity, the cost of ghost signal reducing equipment is prohibitive. Another recently proposed system samples the video signal and the sample is then pseudo-randomly scrambled for transmission. An inverse scrambling operation is performed in the receiver. While this can theoretically yield an increase in ghost performance, the system is quite sensitive to inter symbol interference which produces pseudo random noise that is added to the main image.
The system of the invention utilizes a time dispersal filter that has a variable dispersion factor or coefficient which may be changed to vary the degree of time dispersion of the input signal. In its preferred implementation, the system of the invention utilizes a "hybrid" video signal that has been processed in accordance with copending application Ser. No. 238,956 to remove low frequencies, generally under 200 KHz, which are digitally coded and transmitted as data in the vertical blanking interval of the signal. Thus the processed video signal consists primarily of "edges" with "voids" in the frequency spectrum due to low frequency component removal.
In application Ser. No. 266,740 a technique for time dispersing a timing or clock signal to minimize ghosting and noise in that clock signal is disclosed. The clock signal is processed by a dispersal filter in the transmitter and reconstituted by complementary processing in a dispersal filter of opposite characteristic, i.e. an inverse dispersal filter, in the receiver. Any ghost of the signal is not so processed in the transmitter and is dispersed by the inverse dispersal filter in the receiver. In application Ser. No. 298,081, a signal is processed by a dispersal filter of one sense and the receiver is provided with a dispersal filter that can have its sense changed by operation of a suitable switch. The receiver responds to the transmitted signal and establishes the sense of its dispersal filter to complement that of the transmitter filter. Consequently it is able to receive the transmitted signal. The arrangement is useful for minimizing co-channel interference.
In the system of the invention, the dispersal filter has a variable dispersal characteristic that is varied on a regular basis in response to an algorithm (or reading of a ROM). A dispersal filter in the receiver has its dispersal characteristic varied in a complementary fashion to develop the original signal. The time dispersal characteristic is continually being varied, and multipath signals will be "out-of-step" when processed by the receiver dispersal filter. The result is that multipath or ghost signals are further dispersed in the receiver. Only the main image signal will be reconstituted. The further dispersal of ghost signals will reduce their visual effect to background noise, which is far less objectionable to a viewer than one or more ghost signal images.